Do I understand it correctly that one of the monitors must be connected through HDMI?

I would say either a bad adapter or cable, or bade Thunderbolt port on the Mini, or a bad DP input on the monitor.

Apple says

Adapters for the Thunderbolt 3 (USB-C) or USB-C port on your Mac or iPad Pro - Apple Support

this adapter cable works

Moshi USB-C to DisplayPort Cable - Apple

lllaass is right. The mac mini page lists the adaptors required for what is being connected, a usb-c to displayport cable or adaptor is what is needed here. I would get a straight cable, not an adaptor, sometimes those can be hit or miss. The moshi cable sold by apple doesn't have good reviews and seems flakey. Amazon sells Uni and cable matters cables that have good reviews and are usually much cheaper than what apple sells. This post seems to have had a similar problem with a macbook pro.

https://discussions.apple.com/thread/8017582

I read the specs for your Dell monitor

https://www.dell.com/ed/business/p/dell-u2713hm/pd

and it makes not mention of max of 1080p on HDMI and the 2018 Mini can output 4K@60Hz via HDMI.

You may be limited to 1080p via using an old HDMI cable. The specs do say and HDMI cable is provided with the monitor.

Also, with HDMI connected if you click on scaled in Display Properties what resolutions are available?

Your dongle isn’t compatible with Thunderbolt 3 even though the connector is. If you switch to VGA, your resolution can increase to 2048 x 1152. So find a dongle that works with TB or a dongle for VGA...or stay with HDMI.

The manual makes clear: “HDMI is 1.4, it is compatible and can support up to 1920 x 1080.”

I honestly don’t know anything about dongles but there’s quite a few here who do, let’s hope one chips in. Speaking for myself, I would much rather buy an HDMI 2.0 4K monitor than buy a dongle. But that’s only after I’m able to nitpick about my 1920x1020p connection, has proven impossible thus far but not for lack of trying. Beautiful colors, instant wakes, night-shifting etc. etc. Yes, it’s a Dell.

Fact is, DisplayPort connectors are history in Apple’s world. So for flawless operation it’s either HDMI 2.0 or Thunderbolt 3-certified monitors, including one by Apple to come out later this year. Everything else is, err, a bit dongled.

Yes but...

https://www.amazon.com/gp/customer-reviews/R2N29D4C11QN5Y/ref=cm_cr_othr_d_rvw_ttl?ie=UTF8&ASIN=B01J6DT070

https://www.amazon.com/gp/customer-reviews/R3E9AC13VGB73Q/ref=cm_cr_getr_d_rvw_ttl?ie=UTF8&ASIN=B01J6DT070

https://www.amazon.com/gp/customer-reviews/R2YHS0M65KSN6Z/ref=cm_cr_getr_d_rvw_ttl?ie=UTF8&ASIN=B01J6DT070

All the best...

I’m using two of these to connect my pair of Dell U2715 monitors to a 2018 Mac mini. Not had one issue with these cables.

https://www.amazon.com/gp/product/B07DG2YB5S/ref=ppx_yo_dt_b_asin_title_o08_s00?ie=UTF8&psc=1

When you connected the DisplayPort cable, did you also hook up the USB cable? Did you access the on-screen menu and select the DisplayPort connection as the input? Are using only one video connection at a time?

Did you read the instruction manual?